1. Field of the Invention
The present invention relates to a method of fabricating an electrochemical battery and, more particularly, to a method of injecting the electrolyte into the cell stack of the battery by first placing the electrolyte in a sealed pouch that is placed together with the cell stack into a cell package. The invention also relates to a cell package which can be heat sealed even after it has been contaminated during the process of introducing the electrolyte into the cell stack.
2. Background
A important consideration in the manufacturing of electrochemical batteries is the manner in which the electrolyte is introduced into the cell stack. A current technique includes the steps of pouring the electrolyte into the cell stack during the manufacturing of the cell stack in a machine, placing the electrolyte impregnated cell stack into the cell package, evacuating the cell package and heat sealing the package.
This technique suffers from many disadvantages. The first problem is the loss of electrolyte during the step of pouring the electrolyte into the cell stack and the subsequent step of evacuating the package. It is estimated that between 10 and 30% of the electrolyte is lost during these steps. The electrolyte is a relatively expensive component of the electrochemical cell. Thus, the loss of electrolyte increases the overall cost of manufacturing the battery. Further, since the amount of lost electrolyte cannot be gauged, the final volume of electrolyte that remains in the cell stack is unknown.
A second problem is that the electrolyte that is naturally suctioned from the cell stack during the evacuating step contaminates the inside of the package. Such contamination of the package makes it difficult to securely seal the package. As such, subsequent leakage of the electrolyte from the sealed package may result. A further problem is that the pouring step must be performed in a glove box environment (i.e., dry and inert atmosphere). Since this step is an intermediate step in the manufacturing of the cell stack, the machine which manufactures the cell stack must consequently have a glove box environment, thus driving up the cost of the machine. In addition, when the electrolyte is poured into the cell stack, the electrolyte contaminates the machine thus requiring that it be cleaned on a regular basis. In addition, the vacuum that is applied to the cell stack during the evacuating step may change the solvent ratio of the impregnated electrolyte.
The current cell package is formed of a laminate of a polyester outer layer, an aluminum barrier layer and a polyethylene acrylic acid (EAA) inner layer. The polyester layer provides, strength, the aluminum layer prevents water from penetrating the cell package and the inner layer allows for the heat sealing of the cell package. Specifically, generally, the cell package includes two parts that are bonded together around their periphery by heating sealing the EAA inner layers to each other. The problem with this laminate is that once contaminated with electrolyte, the EAA inner layers cannot form a secure heat seal. This makes degassing and resealing of the cell package a problem. Also, the current material has a relatively high permeability to water necessitating the use of wider heat sealing areas in order to ensure a long shelf life. Finally, the current laminate will absorb electrolyte at elevated temperatures which can interact with the adhesive layer between the EAA inner layer and the aluminum barrier layer.
It is an object of the invention to provide a method of manufacturing an electrochemical battery which overcomes the above problems. In particular, an object of the invention is to provide a method of manufacturing a battery where the electrolyte is introduced into the cell stack with minimal or no loss of electrolyte. Another object of the invention is to provide a method in which the electrolyte filling step is performed after the cell stack is manufactured so that the cell stack manufacturing machine does not have to maintain a glove box environment and contamination of the machine is eliminated.
These and other objects are achieved by a method of fabricating an electrochemical battery, comprising the steps of separately forming an electrode cell stack and a sealed electrolyte pouch, placing the cell stack and the electrolyte pouch together into a cell package, applying a vacuum to the cell package, sealing the cell package, and rupturing the pouch to release the electrolyte into the cell stack. The rupturing step includes the step of squeezing the pouch until the electrolyte squirts out of the pouch. Further, the sealing step includes the step of heat sealing the cell package.
With this fabrication technique there is no electrolyte loss. In particular, since the electrolyte is injected into the electrode cell stack after the package has been sealed, substantially all of the electrolyte is suctioned into the electrode cell stack without any of the electrolyte escaping from the package. In addition, since the electrolyte is not poured into the cell stack during the manufacturing of the cell stack, the cell manufacturing machine does not have to provide a glove box environment. Also, contamination of the machine is avoided. Accordingly, all of the disadvantages discussed above with respect to the current technique are overcome.
Another aspect of the invention is to form the cell package so that it includes two separate enclosures with a tube extending between the two enclosures such that they communicate with each other through the tube. The cell stack is placed in a first one of the enclosures which is then evacuated. The sealed electrolyte pouch is placed in the second enclosure which is also evacuated after the pouch is inserted. Thereafter, as with the previous embodiment, the pouch is ruptured to release the electrolyte from the pouch such that it flows through the tube and into the cell stack contained in the first enclosure. The tube is then sealed and the second enclosure is removed.
Also, with this technique, after formation it is possible to degas the first enclosure. The formation is naturally done after the electrolyte has been released into the cell stack. The degassing step is performed by puncturing the second enclosure and applying a vacuum thereto so that the first enclosure is degassed via the tube. The tube is then sealed and the second enclosure is removed.
It is a further object of the invention to provide a cell package made of a laminate which can be heat sealed even when the cell package is contaminated with electrolyte. This is achieved by a laminate which includes a polyester outer layer, an aluminum barrier layer and a polypropylene inner layer which are adhered together using a unique adhesive which does not break down when contaminated with electrolyte. It has been discovered that the polypropylene layers associated with the overlapped portions of the cell package can be securely heat sealed to each other even when contaminated with electrolyte.